This invention relates to a process for reversibly transferring heat between two zones operating at different temperature levels, and more particularly to a heat pump technique actuated by solar insolation or other low grade heat sources, and to a heat transfer system useful therein.
Numerous techniques have been devised over the past several years for the transfer and/or storage of thermal energy obtained by means of solar collection. Such processes have incorporated chemical heat pumps utilizing thermal energy produced by dissociation reactions (see, e.g., Chubb U.S. Pat. No. 3,972,183), by the dilution of concentrated solutions (see, e.g., Foulke U.S. Pat. No. 4,146,013), by multiple phase changes (see, e.g., Herrick U.S. Pat. Nos. 4,152,899 and 4,154,292), by intermittent absorption cycles (see, e.g., Berg U.S. Pat. No. 4,199,952 and Gilgen U.S. Pat. No. 4,206,745), and the like. Various of these and other solar energy transfer, recovery and/or storage techniques have made use of the distinct thermal characteristics of multiple working fluids upon various phase changes (see, e.g., the above Herrick and Gilgen patents, Bronicki et al U.S. Pat. No. 3,845,628, and Canzano U.S. Pat. No. 4,278,073).
Other methods for the transfer, recovery and/or storage of thermal energy produced by solar insolation are described, for example, in U.S. Pat. Nos. 3,875,926; 4,050,445; 4,052,975; 4,131,158; 4,138,995; 4,182,409; 4,184,477; 4,237,964; 4,258,700; 4,267,825; 4,308,912; 4,341,202; 4,365,661; 4,366,853; 4,375,806; and 4,385,625.
Intermittent cycle chemical effect heat transfer systems have, in particular, been utilized for many years. Hence, prior to 1940 a number of refrigeration devices employing such cycles were commercially marketed, including the so-called "Icyball" unit. In that device a concentrated ammonia/water solution was heated in a "generator" chamber to vaporize the ammonia, the ammonia was cooled and condensed in a condenser chamber or "ball", and subsequently re-evaporated in the ball (with consequent refrigeration effect) and re-absorbed in the generator (see column 2, lines 3-40 of the above Berg U.S. Pat. No. 4,199,952).
Such intermittent cycle techniques have been employed in the conversion of solar energy, not only in the noted Berg patent, but in the above-noted Gilgen U.S. Pat. No. 4,206,745 as well. Gilgen describes a chemical heat pump in which a working fluid consisting of an ammonia/water solution is heated (e.g., by a solar heat source) within a tank to vaporize the ammonia, and the resulting vapor is bubbled into and absorbed by an ammonia/water solution in a second tank utilized as a lower temperature heat sink. Upon cooling of the first tank (e.g., at night) the vapor pressure of the ammonia/water solution therein decreases, creating a pressure differential which produces a vapor flow from the second tank into the first. The vapor is thereupon absorbed by and condensed in the solution in the first tank, the heat of condensation being given up to the solution (and being exchanged, for example, with an appropriate heat exchange medium for use for space heating or the like). The thermal energy of the ammonia vapor is thus transferred to a relatively low-temperature heat sink during the initial (day-time) charging cycle, and recovered during the subsequent (night-time) discharge cycle.
The Gilgen system requires the use of an impractical, immense insulated tank in which the ammonia/water solution is stored. Since both the heat input to, and the heat output from, the Gilgen chemical heat pump is effected by heat exchange with this tank, it is necessary that the tank have a volume sufficient to provide for both the instantaneous heat input and output requirements for the system, as well as a total heat storage capacity sufficient for any desired application. Where, for example, it is desired to utilize the Gilgen heat pump for the storage of approximately 1.times.10.sup.6 B.T.U.'s, it is necessary to employ a pair of insulated tanks, each of which has about a 5,000 to 10,000 gallon capacity. Approximately 40% of the heat transferred by the Gilgen system is transferred by water vapor and is therefore non-recoverable and impedes effective storage. Thus, the Gilgen system is quite limited as to operational flexibility, and imposes substantial capital investment costs.
Moreover, use of the Gilgen heat pump is dependent upon maintaining relatively high differential concentrations of ammonia in the respective tanks (e.g., the formation of an 80% ammonia/20% water vapor mixture at 160.degree. F. is illustrated). However, the concurrent entrainment of water and/or generation of water vapor results in gradual alteration of the required ammonia/water concentrations in the Gilgen tanks, resulting in eventual cessation of all heat transfer capability. When, for example, such a mixture is employed in a Gilgen-type system to meet a heat load of 150,000 B.T.U's per day, absent special techniques for decreasing water-vapor migration or re-establishing the original solution concentrations, the system will cease to function within less than 6 weeks of continual operation. Such a system is thus characterized by markedly impaired heating efficiencies.